1. Field of the Invention
The present invention relates to a method and apparatus for decoding encoded image data, which has been encoded by, e.g., block encoding.
2. Related Background Art
When a document or an image is transmitted or stored, a redundancy reduction code is used to shorten transmission time or to effectively utilize a storage medium. Redundancy reduction encoding is roughly classified into two encoding methods. One method is called a reversible encoding method, and the other method is called an irreversible encoding method.
In the reversible encoding method, data is reversibly encoded. Therefore, loss of information, i.e., deterioration of image quality does not occur. Typical reversible encoding methods are the MH (modified Huffman) and the MR (modified READ) encoding which are used in facsimile systems. The reversible encoding method was developed to process binary data.
Irreversible encoding method include block encoding, vector quantization, orthogonal conversion encoding, and the like. The assignee of the present invention has proposed the irreversible encoding method in U.S. Pat. No. 4,797,945. However, when the irreversible-encoded data is decoded, information is normally lost, resulting in deterioration of image quality.
Deterioration of image quality in the irreversible encoding method does not occur in a region where variations in gray level or tone are small, but noticeably occurs in regions where variations in gray level are large, e.g., an edge portion of a character. As a result, an edge of a character is made unclear or blurred. In a region where variations in gray level are small, a block becomes conspicuous, and gradation visibly deteriorates.